This invention relates to a carriage assembly for use with an optical recording disc for positioning an optical lens for optical recording and reproduction relative to the disc.
FIGS. 1 and 2 illustrate, in perspective, an example of a conventional carriage assembly for use in an optical disc apparatus.
Such a conventional carriage assembly comprises a frame 1, an optical lens 2 for projecting and receiving a light beam for the optical recording and reproduction relative to an optical disc 3 which rotates about a shaft 3a, and a carriage 4 movably supported in the frame 1. The carriage 4 comprises a translating member 4a of substantially rectangular cross-section having a data converter unit (not shown) incorporated therein, and a pickup assembly 4b mounted on one end portion of the translating member 4a, the pickup assembly having mounted the objective lens 2 thereon. The assembly further includes a radial guide arrangement 7 for permitting the carriage 4 to move in a radial direction relative to the disc 3. The radial guide arrangement 7 comprises linear bearings 8 mounted on both sides of the translating member 4a of the carriage 4 and a pair of guide rails 9 secured to the frame 1 in a parallel relationship with respect to each other in the radial direction of the disc 3. The linear bearings 8 engage the guide rails 9 such that the carriage 4 moves along the guide rails 9. The carriage assembly further comprises a radial drive arrangement 10 including an electromagnetic coil 11 secured to the translating member 4a of the carriage 4 and a pair of magnetic circuits 12. Each of the magnetic circuits 12 comprises an outer stator yoke 13 of a magnetic material having a substantially U-shaped horizontal cross section, a permanent magnet 14 disposed in an inner side of the bight portion of the U-shaped outer stator yoke 13 and a center pole piece 15 connected to the ends of the leg portions of the U-shaped outer stator yoke 13, through which a magnetic flux flows. The magnetic circuits 12 are electromagnetically coupled with the electromagnetic coil 11. The optical lens 2, the carriage 4, the linear bearings 8 and the electromagnetic coil 11 together constitute a moving unit. The guide rails 9, and the magnetic circuits 12 comprising the U-shaped yokes 13, the permanent magnets 14 and the center pole pieces 15 together constitute a stationary unit.
Referring now to FIG. 2, the pickup assembly 4b includes a vertical drive arrangement 20 and a radial drive arrangement 30 for positioning the optical lens 2 at a desired position on the optical disc 3 to project light beam thereon.
The vertical drive arrangement 20 comprises a cylindrical stator yoke 21, a cylindrical stator permanent magnet 22 mounted on the cylindrical stator yoke 21 with a predetermined gap therebetween, a cylindrical drive coil 23 movably mounted between the cylindrical stator yoke 21 and the magnet 22, a rod 26, one end of which is fixed to the center of the stator yoke 21 and the other end coaxially extending centrally upwards through the cylindrical stator permanent magnet 22 and the cylindrical drive coil 23, a sub cylindrical stator yoke 21a which is coaxially mounted on the stator magnet 22, a sleeve 25 slidably mounted on the rod 26, a turn-table 24, having mounted the optical lens 2 thereon, connected at the center thereof to the upper end of the sleeve 25, and a stopper plate 27, connected to the other end of the rod 26, provided for preventing the turn-table 24 from coming off the rod 26.
The radial drive arrangement 30 comprises a pair of radial drive coils 31 mounted on peripheral edges of the turn-table 24 in an opposite relationship with respect to each other, and a pair of stator permanent magnets 32 disposed on the sub cylindrical stator yoke 21a in the vicinity of the respective radial drive coils 31 through plates 33 of a non-magnetic material. Each of the radial drive coils 31 is electromagnetically coupled with a correspondiing stator permanent magnet 32, a pair of springs 34 disposed between the turn-table 24 and the optical lens 2 so as to maintain the lens 2 at a predetermined position.
When the electromagnetic coil 11 is energized in accordance with a command from an external control unit (not shown), a magnetic field is generated around the electromagnetic coil 11. The magnetic field generates a driving force in cooperation with the magnetic flux flowing through the respective center pole pieces 15 as defined by Fleming's rule, whereby the carriage 4 together with the objective lens 2 is moved in the direction indicated by an arrow A or A' shown in FIG. 1, i.e. in the radial direction of the disc 3. The objective lens 2 mounted on the pickup assembly 4b is arranged so as to be movable in the radial direction and/or in the vertical direction of the disc 3 along the optical axis thereof by means of the vertical drive arrangement 20 and the radial drive arrangement 30 respectively, thereby allowing the light beam projected from the objective lens 2 to be positioned at a desired position on the disc surface.
When recording or reproducing function is to be performed, first the objective lens 2 is approximately positioning over a desired area on the disc 3 by regulating the current supplied to the electromagnetic coil 11. Once the lens 2 is positioned over the desired area of the disc 3, the current supplied to the electromagmetic coil 11 is cut off, whereby the driving force ceases. Then, the objective lens 2 is moved in the radial and/or perpendicular directions of the disc 3 by means of the vertical drive arrangement 20 and the radial drive arrangement 30, by controlling the current flowing through the electromagnetic coils 31 and the cylindrical electromagnetic coil 23, respectively, so that the light beam projected through the objective lens 2 is precisely positioned at a desired position on the disc surface.
The thus-constructed conventional carriage assembly has problems in that, in order to position the objective lens 2 at a desired position over the disc surface, the pickup assembly 4b must have a vertical drive arrangement 20 for driving the objective lens 2 in the vertical direction of the disc 3 to adjust the focus of the lens 2 and a radial drive arrangement 30 for moving the objective lens 2 in the radial direction of the disc 3 by the diametrical movement of the turn-table 24 of the pickup assembly 4b. Accordingly, the carriage assembly is both cumbersome and weighty and the moving speed of the carriage 4 cannot be increased, resulting in a reduced access time of the objective lens 2. The physical dimensions of the moving unit are large, so it becomes difficult for the carriage assembly to be made compact. Further, the magnetic circuit 12, which comprises the outer stator yoke 13, the permanent magnet 14 and the center pole piece 15, has no magnetic flux path in the vertical direction, so magnetic flux leaks out in the vertical direction.